In such devices, a generally multi-layer substrate carries chromophore elements that are chemical molecules or biological molecules, dye elements added or grafted to the chemical biological molecules, or semiconductor nanostructures such as quantum boxes or wires that respond to excitation by appropriate light by emitting fluorescence at a wavelength that depends on their nature. This fluorescence makes it possible in particular to identify and find on the substrate molecules that have responded to given treatments.
Proposals have already been made in patent applications WO-A-02/16912, FR 01/15140, and FR 02/10285 in the names of the same inventors, for means enabling the excitation intensity of chromophore elements to be reinforced, and thus to reinforce the fluorescence emitted by said elements. In particular, it has been envisaged to use a guided mode for the excitation light in order to excite the chromophore elements from the evanescent portion of the guided wave, which is advantageous in terms of signal-to-noise ratio and in terms of excitation light rejection. In the known technique, it has been proposed to couple the excitation light into a guiding layer of the substrate, either by illuminating a grating formed on the surface of said guiding layer, or by eliminating an edge of the guiding layer with the excitation light.
In the first circumstance, coupling is achieved only if the excitation light reaches the grating at a angle of incidence that is very accurately determined, with said angle varying with the wavelength to be coupled. The equipment therefore needs to be set up with very great mechanical and optical accuracy. In the second circumstance, the beam illuminating the edge of the guiding layer must be of a dimension that corresponds to the effective height or width of the guided mode, which is 1 micrometer (μm) or less, and it must therefore be positioned with tolerance of less than 1 μm.
The practical difficulties associated with coupling excitation light into the guiding layer of the substrate are therefore considerable.